1. Field of the Invention
This invention relates generally to polyurethane foams. More particularly, this invention relates in one embodiment to polyurethane foams that are thermoformable at elevated temperatures, and have good low temperature and/or room temperature compression set resistance. In another embodiment, this invention relates to thermoformable polyurethane foams having moisture management properties. Such thermoformable and moisture managing foams may be used in single or dual layer constructions.
2. Description of the Related Art
Conventional polyurethane foams, for example the PORON® foams manufactured by Rogers Corporation, may be formulated to provide an excellent range of properties, including compression set resistance, even at higher temperatures. Foams with good compression set resistance provide cushioning, and maintain their original shape or thickness under loads for extended periods. Most polyurethane foams are thermosetting compositions, and are therefore not thermoformable, meaning they cannot be molded or formed at temperatures below that at which degradation occurs. In contrast, typical thermoformable materials can be shaped at elevated temperatures, but exhibit poor compression set resistance even at room temperatures. Products made from these materials have a shorter product life over which the cushioning and ability to maintain their initial shape is good.
Known thermoformable polyurethane foams suffer from the same drawbacks. One method for obtaining a thermoformable polyurethane foam is by the addition of powdered, rigid, thermoplastic polymers to polyurethane froth foams. U.S. Pat. Nos. 5,432,204 and 5,668,189, for example, teach that thermosensitive fillers and microwave sensitive powders impart some thermoformability when added to foam prepared from polyisocyanate, water, catalyst, and polyol.
U.S. Pat. No. 4,508,774 discloses cellular polyester-urethane foams having a density of between 15 to 400 Kg/m3 (0.9-24 lb/ft3) formed by pressing an initial foam having a density of between 15 to 40 Kg/m3 (0.9-2.4 lb/ft3) at temperatures of between 140° C. and 200° C. The initial foam is the reaction product of a mixture of diphenylmethane diisocyanates and polyphenyl-polymethylene polyisocyanates with a polyester polyol under foaming conditions. The polyester polyol is required to be at least 55 percent by weight of the polyol component. Due to its high degree of sound absorption, the foam is shaped into panels and used for headliners and trim panels in automobiles.
U.S. Pat. No. 4,757,096 discloses the formation of polyurethane foam containing either an aromatic amine or an alkoxylated bisphenol, a chain extender that imparts the property of thermoformability to a cured polyurethane froth foam. The shape retention of the foam is enhanced with the addition of a second chain extender, but the thermoformability is then compromised.
Conventional hydrophilic or moisture managing foams are typically formulated from a hydrophilic polyurethane formulation and large amounts of water used for blowing. Typically these foams have very poor physical properties and compression set resistance, which renders them ineffective as cushioning materials. In contrast, polyurethane foams, for example the PORON® foams manufactured by Rogers Corporation with very high compression set resistance and excellent cushioning properties typically exhibit low water absorption.
Hydrophilic, flexible polyurethane foams that retain a compressed form are disclosed in U.S. Pat. No. 6,034,149. The foam is made of water reacted with a prepolymer. The prepolymer is made by reacting excess polyisocyanate with a polyether polyol, the polyol having an average nominal hydroxyl functionality of 2 to 6. These hydrophilic foams are used as diapers, sponges, and wound dressings. U.S. Pat. No. 6,034,149 does not teach how to attain thermoformable foam having both thermoformability and shape retention and good compression set resistance properties.
Composite materials for absorbing and dissipating body fluids and moisture are disclosed in U.S. Pat. No. 5,763,335. The foam layer is established by combining an aqueous mixture including the sorbent in a pre-determined ratio with the hydrophilic polyurethane. U.S. Pat. No. 5,763,335 teaches generally hydrophilic foam, but does not teach how to attain a moisture-managing foam that has good compression set resistance and comfort properties.
Polyurethane foams having a combination of thermoformability at elevated temperatures and excellent compression set resistance at room and/or low temperatures, especially together with good shape retention under load or elevated temperature, have not yet been achieved. Similarly, thermoformable foams that are also moisture managing and have excellent cushioning and compression set resistance, have not yet been achieved. Moisture management in this case is defined as the ability of the foam to absorb water (greater than or equal to 50% of its own weight when immersed in water as per ASTM D570) in conjunction with the ability to give up the absorbed water or dry to less than or equal to 20% of the absorbed water in less than or equal to 24 hours. There is accordingly a present need for a polyurethane foam that is both thermoformable with or without moisture managing characteristics and has good compression set resistance, and especially together with shape retention properties.